1. Field of the Invention
This invention relates to the production of integrated circuits, and, more particularly, to an apparatus for ultrasonically bonding external leads of a semiconductor integrated circuit to electrodes of the integrated circuit.
2. Background Information
Wirebonding is the most common technology used in the electronic packaging industry for making electrical connections between the electrode pads on an integrated circuit chip pellet and wires extending to a substrate providing external connections to the chip. Wirebonding is typically performed using ultrasonic bonding, thermocompression bonding, or thermosonic bonding. While ultrasonic wedge wirebonding is slower and more expensive than the other types, it currently becomes more popular because it can be performed at a lower temperature.
In ultrasonic wirebonding, the wire is typically fed through a hole extending through a bonding tip. The hold may extend centrally and axially downward through the tip, or it may extend at an oblique angle from an opening at a side of the tip to an opening in the distal surface of the tip. With conventional apparatus, ultrasonic vibrations are introduced into a proximal end of the bonding tip by means of an ultrasonic transducer through an ultrasonic horn. During the bonding process, the wire, extending from the distal end of the bonding tip, is pressed between the distal end of the bonding tip and the electrode of the circuit chip pellet. Thus, the bonding tip transmits ultrasonic vibrations between the ultrasonic horn and the wire, holds the wire in place, and presses the wire against the electrode, as required in the bonding process. With the wire firmly held in place, vibrations are applied at a typical frequency of 60 KHz with an amplitude of 2.5 microns.
In conventional ultrasonic wirebonding apparatus, vibrations are derived from the operation of a magnetostrictive transducer including a laminated metallic core composed of a magnetostrictive material, such as nickel, an excitation coil, to which an alternating frequency current is applied to produce the vibrations, and a polarizing coil, to which a direct current is applied. These vibrations are fed through an ultrasonic horn or coupling to the bonding tip, at which vibrations are transmitted to the wire being bonded. While such a transducer is readily able to produce the type of vibrations needed for wirebonding, it is necessarily large in size and mass.
Alternatively, the vibrations needed for wirebonding are conventionally produced using a stacked rectangular piezoelectric transducer. What is needed is a combination of an ultrasonic transducer which is small and light, and which has a center of gravity physically near the point at which bonding occurs, and a drive means which is also light and capable of rapid response.
Between wirebonding operations on an individual chip, either the wirebonding apparatus or the chip must be moved by a servomechanism, in order to traverse between the points at which wirebonding is to occur, and in order to bring the bonding tip into contact for the bonding process and out of contact for movement to the next point. If the wirebonding apparatus is to be moved, the wire, the bonding tip, and the ultrasonic transducer must be moved together, creating a significant mass which severely limits the speed at which the apparatus can be moved. This configuration physically separates the center of mass of the transducer from the distal end of the bonding tip, which must be accurately located at each bonding site, increasing the complexity of the dynamics associated with movement of the bonding tip. While the circuit chip being carried through the wirebonding process is much lighter than the wirebonding apparatus, moving the chip to traverse between wirebonding points while the wirebonding apparatus is held stationary places further burdens on the manufacturing process, since each chip must be loaded in a fixture to be moved separately, chips cannot be processed in a batch or as a part of a continuous flow.
Piezoelectric (piezoceramic) tube actuators have been used in a number of applications requiring the precise control of very small motions, such as the micropositioning for scanning tunneling microscopy.
What is needed is a wirebonding system in which circuit chips may be handled by means of a batch or continuous process, with the required movements among points for wirebonding being handled by moving the wirebonding apparatus, as the circuit chip is held still. Such a system needs a very light, and hence easily movable method for generating ultrasonic vibrations. It is also very desirable that the transducer producing the vibrations is located physically close to the distal end of the bonding tip, which must be accurately moved.